Fix Docker::Image.create

When pulling a private Image hosted by etiher [Docker's Index](index.docker.io) or [Quay](quay.io), `docker-api` could not pull the image because the proper headers were not being set. This pull ensures `Docker::Image.create` sets those headers, and adds a test.

@tlunter Could you take a look at this?
@windowsrefund This pull should fix your issue.